Electrodeposition of a ternary alloy of nickel, iron and molybdenum



3,271,274 ELECTRUDEPQSHTEGN @F A TERNARY ALLUY 0F NICKEL, IRQN AND MOLYEDENUM Guy Di Guiiio, Philadelphia, Pa, .ioseph S. Mathias,

Riverton, NJL, and Edwin F. Schneider, Jenkintown, Pa, assignors to Sperry Rand Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Get. 31, 1962, Ser. No. 234,518 8 Claims. (Cl. 204-43) The present invention relates generally to an improved magnetic fi-lm particularly adapted for use as a memory element in a data processing system, and more particularly to an improved technique for electrolytically depositing magnetic films of this type which consist essentially of a ternary alloy of nickel-iron-molybdenum.

In the preparation of magnetic films which are designed for use in memory applications with data processing systems, it is generally desirable that the films exhibit properties which permit switching at the lowest reasonable energy requirement possible, the energy level being consistent with the stability of the memory film or core. It is also generally desirable that the film have a composition which provides for a minim-a1, if any, magnetostriction, and in addition it is generally preferable that the magnetocrystalline anisotropy be substantially zero. It has been found that memory films having a very low or substantially zero co-eificient magnetostriction are desirable from the standpoint of stability, consistency, and uniformity in switching, and it has been further found that there is more freedom of physical arrangement of film devices if the magnetocrystalline anisotropy is zero. Films which have little or no magnetocrystalline anisotropy also have been found to have low angles of dispersion. While the specific reason for the dispersion characteristics are not fully understood or appreciated, it has generally been found that it is possible to prepare a better quality memory film when the dispersion angle is low.

While it has been proposed in the past to prepare electroplated nickel-iron-molybdenum films for magnetic datastorage devices, these films have normally had exceedingly high values of l-I this figure representing the value of the anisotropy field. The values of transverse field required for rotational switching are a function of H and accordingly for memory devices, reasonably low H values are preferred. In accordance with the technique of the present invention, improved electroplated nickeliron-molybdenum magnetic data storage devices may be prepared which have very low H values, these values being sufiiciently high, however, to preserve the memory characteristic or capability of the films. In this regard, the optimum values of H normally lie between 1.0 and 2.0 oersteds, this value permitting rotational switching of the core at a relatively low magnitude of energy with the entire switching operation being accomplished with a lower overall energy requirement than would otherwise be necessary with films having significantly higher H values. In addition, the H values are sufiiciently high so that the films are not readily disturbed by stray magnetic fields of modest or moderate intensity. The system aspects are accordingly not adversely affected.

In addition to the specific values of the anisotropy field H, the H values are likewise important, this figure relating to the coercive field of the film. Ideally, for most switching operations, the value of H should be somewhat less than value of I-I the range of the H /H ratio values preferably being 0.5 and 1. With this particular ratio range, and with presently available switching equipment, switching techniques utilized and the like, it has been found that the switching characteristics of a film having these characteristics are very useful. When the films are inverted, that is, the value of H exceeds that nited States atent value of H the films produced have been found to generally exhibit poor rotational switching characteristics. In addition, it has been generally found that poor dispersion exists in inverted films,

In accordance with the technique of the present invention, an improved electroplating technique is provided for the formation of thin nickel-iron-molybdenum films. The most important feature of the invention is the provision of a critical range of current density values which may be employed in order to prepare the improved nickel-iron-m-olybdenum films, these films having low and controlled values of H, together with optimum values for the ratio of H At the present time, when conventional solutions and conventional TI current density values are utilized to prepare electroplated nickel-iron-molybdenum film, the resultant magnetic members have been found to have unusually high H values, these values being substantially higher than those achieved in connection with the present invention and ranging up to about 5 oersteds.

In performing the technique of the present invention, a plating solution is prepared including nickel sulfate, ferrous sulfate, together with sodium molybdate, these materials being included in an acid plating bat-h along with certain conventional plating bath additives. These additives including the citric acid, saccharin, sodium lauryl sulfate, and sodium chloride are considered as conventional additives only, these being utilized in order to enhance the plating characteristics of and to control the ultimate plating of the film, and to otherwise control or modify various characteristics of plating. These additives are conventional in the planting art and do not in and of themselves provide a part or portion of the present invention, and it will be appreciated that other specific plating additives may be employed in order to achieve the results of the present invention. The current density employed ranges from bet-ween 6 and 1 2 ma./cm. and preferably is about 7 to 10 ma./cm. While the current density has been found to essentially control the rate at which the deposit is formed upon the substrate, it has been further found that the magnitude of current density has an affect upon the values of H along with the values of the ratio H /H Utilizing the techniques of the present invention, films are formed which have appropriate values of H and H making them partic ularly adaptable for use as magnetic memory elements in data processing systems.

Therefore, it is an object of the present invention to provide an improved electroplating method for the formation of thin nickel-iron-molybdenum films, these films having controlled and desirable values of I-I and H /H ratios, these elements being particularly valuable for use as magnetic films for certain memory applications in data processing systems.

It is a further object of the present invention to provide an improved electroplating method for the formation of thin nickel-iron-molybdenum films wherein a certain critical current density is employed for controlling the rate of deposition of electrodeposit to an optimum de gree.

It is yet a further object of the present invention to provide an improved elecrtoplating method or technique for the formation of thin nickel-iron-molybdenum films which are particularly adaptable for use in magnetic memory applications, the technique employing the use of an acid electroplating bath including nickel-iron- -and molybdenum, and being particularly adaptable for use in plating at a certain preferred range of current density.

Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification and appended claims.

In accordance with the preferred modifications of the present invention, an electroplating bath is prepared having a composition range as is indicated herembelow:

NiSo 6 H O ig./ l 60-80 Citric acid g./l 70-100 FeSO -7H O g./l 2.5-4 Sodium molybdate g./l 1.252.0 Saccharin g./l 0.80-1.0 Sodium lauryl sulfate g./l 0.2-0.6 NaCl g./l 10.0

pH of the bath 3 to 6 Temperature C- 25-30 Current density ma./cm. 6-12 Percentage Element: composition Nickel 60 to 77 Iron 13 to 24 Molybdenum to 16 In one typical electroplating operation, a solution having the following composition was employed:

NlSO46H2O g./l Citric acid g./l 72 FCSO4'7HZO g./l 3.5 Sodium molybdate -g./l 1.5 Saccharin -g./l 0.8 Sodium lauryl sulfate g./l 0.2 NaCl g./l 10 pH of the bath 5.2 Temperature C 25 A current density of 6 ma./cm. Was utilized in connection with this bath, and under these conditions the following data is obtained:

Film Time II, (0e) Hi (00.) Thitirness,

50 sec 2. 2 2.0 700 70 sec... 1.6 1. 4 1, 000 90 sec 1. 8 1. 7 1, 300 150 sec. 1. 1 1. 9 2, 000

The composition of the films prepared in accordance with this specific example was typically as follows:

Percentage Element: composition Nickel 63.0 to 72 Iron 18.0 to 23.0

Molybdenum 10.0 to 14 Referring now to the individual ingredients in the plating bath, it will be appreciated, of course, that the anion of the nickel and iron salt and the cation of the molybdate salt is not particularly critical, it being appreoiated that the ratios of the materials in the bath are critical. In this regard, the *Fe++/Ni++ concentration ratio ranges from between about 0.03 up to about 0.06, based upon the normalities of these ions in solution. With regard to the concentration ratio of the molybdate ion to nickel, based upon normalities, the ratio M0O -/Ni++, ranges from between about 0.016 up to about 0.036. It will be appreciated that the various concentration ranges set forth herein will normally enable one to obtain the electroplated thin films in accordance with the present invention, these films having the desirable properties of magnetic thin films for memory applications.

With regard to the normality terms used herein, a one N solution of Fe++ includes one-half gram-molecularweight of the salt, for example, FeSO a one N solution of nickel Ni++ includes one-half gram molecular-weight of the salt, for example, NiSO a one N solution of MoO rincludes one-half gram-molecularweight of the salt, for example, Na MoO While the molybdenum is ultimately reduced from an oxidation state of 6 to the free metal, the normalities as expressed herein do not take account of the ultimate plating reaction through which the molybdenum goes. In other words, the term normality as used herein relates solely to the concentraction of the various ions of the individual salt solutions with sole reference being to the salt ions per se.

With regard to the moly'bdate, it will of course be appreciated that the particular cation employed should not be a platable cation. For this purpose, any of the alkaline earth metals such as sodium moly-bdate, potassium molybdate, or the like may be utilized.

The status of the pH of the bath has been indicated hereinabove. It will be observed that the bath is mildly on the acid side, this being desirable for plating the film-s in accordance with the technique of the present invention. The temperature range is not particularly critical, and it will be appreciated that good results may be achieved with holding the bath at substantially or near room temperature.

With regard to the current density employed, it will 'be observed that the range of between 6 and 12 ma./cm. has been indicated. It will be appreciated that if a current density substantially lower than the minium indicated is employed no deposition Will occur. If, on the other hand, a current density is employed which substantially exceeds the maximum indicated hereinabove, films having high coercivity with distorted loops will develop. Accordingly, the range indicated is a preferred range, and in particular, a current density of about 6 ma./cm. is preferred.

The substrate employed is preferably an insulating substance such as glass or plastic, ordinary micro-slide glass being preferred. The surface of the substrate is initially cleaned of all contaminants and is preferably polished to present a smooth plating surface. The substrate is then coated with a layer of gold or chromium-gold, the latter including a pair of layers wherein an initial layer of chromium is applied followed by the aplication of a layer of gold. 'For example, the substrate surface treated may employ a plurality of individually spaced circular plated areas of a combined coating of about A. of chromium followed by 100 A. of gold. Conventional evaporative techniques are employed to coat the substrate. In lieu of an insulated substrate, it is, of course possible to employ a polished metallic surface as a substrate, if desired.

When the use as a magnetic memory core is anticipated, it is, in certain instances, desirable that the film be produced in the presence of an external magnetic field, this field being applied during the plating operation. The application of such a magnetic field enhances the uniaxial anisotropy characteristics of the film. In this connection the field will be applied to the film being plated along the plane of the film. The field preferably is a strong field having a strength of about 25 to 35 oersteds.

It will be appreciated that the specific examples listed hereina'bove are provided for purposes of illustration only and are not to be otherwise construed as a limitation upon the scope of the present invention. It will be further understood, therefore, that those skilled in the art may depart from specific examples without actually departing from the spirit and scope of the present invention.

What is claimed is:

1. The method of electrolytically depositing thin magnetic nickel-iron-mo'lybdenum films having the composition in the range 6077%, 13-24% and 16%, respectively, from an aqueous plating bath including Ni++, [Feand M005", said method being particularly characterized in that the Fe++/Ni++ normality ratio in said bath ranges from about 0.03 up to 0.06, the MoO. -/-Ni++ normality ratio in said bath ranges from about 0.016 up to about 0.036, the pH range of said bath is maintained between about 3 and about 6, and the current density employed with said bat-h ranges from about 6 ma. up to about 12 ma. for each cm. of plating surface.

2. The method as set forth in claim 1 being particularly characterized in that the current density ranges from between 7 ma. up to 10 ma. for each cm. of plating surface.

3. An aqueous acidic plating bath for preparation of electrolytically deposited nickel-iron-molybdenum films having the composition in the range 60-77%, 13- 24% and 10-46%, respectively, said bath consisting essentially of from about 0.45 N up to about 0.06 N in =Ni++, from about 0.018 N up to about 0.029 N in Fe++, and from about 0.01 N up to about 0.017 N in M0O.f

4. The plating bath as set forth in claim 3 being particularly chracterized in that the pH is between about 3 and 6.

5. The method of electrolytically depositing thin magnetic nickel-iron-molylbdenum fi l-ms having the composition in the range 6077%, 13-24% and 1016%, respectively, from an aqueous plating bath including Ni++, Pe and M005 salts, said method being particularly characterized in the Fe++/ Ni++ normality ratio in said bath is about 0.055, the MoO /Ni++ normality ratio in said bath is about 0.027, the pH range in said bath maintained between about 3 and about '6, and the current density employed in said bath ranges from about 6 up 6 to 12 ma. for each square centimeter of plating surface.

6. The method as set forth in claim 5 being particularly chracterized in that the current density ranges from between 7 ma. up to 10 ma. for each cm. of plating surface.

7. The method as set forth in claim 5 being particularly characterized in that an external magnetic field is applied to the electrolytically deposited thin magnetic nickel-iron-molybdenum film being plated, said external magnetic field being applied along the plane of said film.

8. The method of electrolytically depositing thin magnetic nickel-iron-molybdenum films having the composition -77%, l3-24% and 10-46%, respectively, from an aqueous plating bath including Ni+ Fe++ and Moo -1 said method being particularly characterized in that the Fe++/Ni++ normality ratio in said bath ranges from about 0.03 up to 0.06, the M-oO -/Ni++ normality ratio in said bath ranges from about 0.016

up to about 0.036, the pH range of said bath is maintained between about 3 and about 6, the current density employed in said bath ranges from about 6 ma. up to about 12 ma. for each cm. of plating surface and wherein an external magnetic field is applied to the film being plated along the plane of the film.

References Cited by the Examiner UNITED STATES PATENTS 2,599,178 6/-l9*5'2 Holt et al. 204-43 3,027,309 3/ 1962 Stephen 20443 3,032,485 5/1962 Tsu et al. 20443 OTHER REFERENCES Iron-Nickel Alloys for Magnetic Purposes, The International Nickel Company, Inc, p. 18, 1953.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

8. THE METHOD OF ELECTROLYTICALLY DEPOSITING THIN MAGNETIC NICKEL-IRON-MOLYBDENUM FILMS HAVING THE COMPOSITION 60-77%, 13-24% AND 10-16%, RESPECTIVELY, FROM AN AQUEOUS PLATING BATH INCLUDING NI++,FE++ AND MOO4--, SAID METHOD BEING PARTICULARLY CHARACTERIZED IN THAT THE FE++/NI++ NORMALITY RATIO IN SAID BATH RANGES FROM ABOUT 0.03 UP TO 0.06, THE MOO4--/NI++ NORMALITY RATIO IN SAID BATH RANGES FROM ABOUT 0.016 UP TO ABOUT 0.036, THE PH RANGE OF SAID BATH IS MAINTAINED BETWEEN ABOUT 3 AND ABOUT 6, THE CURRENT DENSITY EMPLOYED IN SAID BATH RANGES FROM ABOUT 6 MA. UP TO ABOUT 12 MA. FOR EACH CM2 OF PLATING SURFACE AND WHEREIN AN EXTERNAL MAGNETIC FIELD IS APPLIED TO THE FILM BEING PLATED ALONG THE PLANE OF THE FILM. 